The present invention relates to roll-type pressure developers or presses in which one of the pressure rolls contains a load-bearing composite material.
In multiple roll pressure developers or presses, a pair of pressure rolls are commonly used for the application of a uniform pressure to a sheet or web, such as a sheet of paper. The pair of pressure rolls may be shaft-loaded, or may be loaded by an external force applied by one or more additional loading rollers. Roll-type developers are commonly used in the application of a uniform pressure to paper stock across a nip. For example, pressure rolls are commonly used in the application of a uniform pressure to fix images on paper or to rupture the microcapsules of an imaging material, such as that shown in commonly assigned U.S. Pat. Nos. 4,399,209 and 4,440,846. In order to achieve these results, relatively high pressures must be exerted through the pressure rolls on the paper stock. For example, pressures of about 6000 to 8000 lbs/in.sup.2 are required for rupturing microcapsules. In designing pressure rolls useful in the application of such tremendous pressure, materials are selected which are capable of performing under such conditions.
In the past, pressure rolls have been made of metallic materials such as steel in order to provide sufficient strength. For example, see U.S. Pat. Nos. 4,426,411 and 4,521,184.
Pressure rolls have also been made of metallic materials with release coatings thereon. For example, U.S. Pat. No. 3,776,760 teaches a fuser roll comprising a roll having a tetrafluoroethylene coating thereon. U.S. Pat. No. 3,942,230 teaches a roll comprising a body member of a base metal, a porous plate of a nickel chromium alloy, and a release film of a fluorocarbon polymer.
Although metallic pressure rolls and coated metallic pressure rolls have been used satisfactorily, such rolls are undesirable because the metallic materials, especially steel, are heavy and expensive.
U.S. Pat. No. 4,372,247 teaches a fusing apparatus which includes a pair of rolls. Each pressure roll comprises a cylindrical metal tube which defines the roll exterior. A cured elastomer such as a urethane elastomer is bonded to the tube interior. The elastomer is also bonded to a support shaft extending through the tube. The fusing apparatus was designed to overcome the disadvantages such as paper wrinkling of a skewed roll apparatus while providing uniform pressure across the nip. However, such a roll may not be sufficiently rigid due to the elastomeric internal material.
A further inherent disadvantage of the system shown in U.S. Pat. No. 4,372,247 is the fact that it will not operate satisfactorily at the high pressures required to develop images in which microcapsules are ruptured, as stated above. The reason for this that a pair of identical pressure, as disclosed, will tend to have identical bending moments at the same axial locations. The deflection curves of a pair of nip-defining rolls can be matched only when the bending moments of the rolls compliment each other, not duplicate each other.
Although improvements in pressure rolls have occurred, a need exists for a pressure roll which is light weight, inexpensive, and has a low scrap rate at production.